1. Field of the Invention
The present invention relates to an ink set and an ink jet recording method.
2. Description of the Related Art
Inks containing pigment as coloring material have been widely used in ink jet recording methods because of the excellent color saturation and fastness properties of the recorded images. Furthermore, the ink jet recording method must be able to record various images, including letters, tables and figures, photographs, and pictures, on a wide range of recording media such as plain paper and ones having a porous layer, e.g., glossy paper and art paper. The characteristics the inks should have are different depending on what kind of recording medium is used; thus, inks in which pigment is dispersed by different methods have been used for different purposes. For example, when glossy paper or a similar recording medium is used, inks containing a pigment dispersed with a resin dispersant are chosen because the use of such inks advantageously improves the glossiness, the abrasion resistance, and other characteristics of the recorded images. When the recording medium is plain paper, art paper, or the like, inks containing a “self-dispersible” pigment are chosen because this type of ink advantageously improves the optical density and the color saturation of the recorded images.
Researchers are seeking ways to make pigments such as carbon black and organic pigments self-dispersible, and the most widely used self-dispersible pigment is carbon black as a component of black inks. The self-dispersible type carbon black, which is better than the self-dispersible type organic pigment in regard to masking properties and color developability, is suitable for use as coloring material in inks for ink jet.
Some previous research has studied the addition of a resin particle to an ink containing a self-dispersible type carbon black. For example, Japanese Patent Laid-Open No. 2002-265831 proposes an ink containing a self-dispersible pigment and a resin particle whose average particle diameter is within ±50 nm of that of the pigment, and this improves the abrasion resistance of the images recorded using the ink and makes the ink less likely to clog the recording head used therewith. Japanese Patent Laid-Open No. 2010-132908 proposes an ink containing carbon black and a resin particle whose content is 20 times or more of that of carbon black, and the use of this ink reduces the metallic luster of images caused by reflected light. Furthermore, Japanese Patent Laid-Open No. 2007-051176 proposes an ink set including a black ink and a color ink. In this ink set, the black ink contains a self-dispersible pigment and a resin particle, and the color ink contains an organic pigment and a resin particle.
Extended-life of an ink jet recording apparatus with an increased yield has been in demand because of the recent trend toward resource saving. After extensive research on inks containing a self-dispersible type carbon black as coloring material, the inventors found that long-term ejection of such an ink causes the direction of ejection of the ink to deflect and the ink not to travel straight (irregular ejection due to continuous use). Adding a resin particle to the ink in attempt to solve this problem improved the irregular ejection due to continuous use but not to a sufficient extent. Furthermore, recording by an ink containing a self-dispersible type carbon black and a resin particle in combination with an ink containing a resin caused more significant irregular ejection (irregular ejection due to contact between inks). It is strongly recommended that any type of irregular ejection be immediately resolved because such a defect causes the ink(s) not to be applied to the intended points on the recording medium, affecting the quality of the resulting images. These problems, i.e., two types of irregular ejection, occurred regardless of the method used to eject the ink(s) or the configuration of the recording head.